1. Field of the Invention
The present invention relates to injection molding machines and, more particularly, to an injection unit, as part of an injection molding machine, which has two hoppers for plastic raw material adapted to be alternatingly moved into an operating position in which plastic granulate is fed from the hopper into the injection unit.
2. Description of the Prior Art
It is known from the prior art to arrange a granulate hopper on the injection unit of an injection molding machine in such a way that the hopper is slidable from its operating position to a discharge position along a horizontal transverse guide plate. Such a hopper arrangement is disclosed in my German Pat. No. 30 46 387 and in my corresponding U.S. Pat. No. 4,418,845.
This prior art hopper arrangement features a flat transverse guide plate which is bolted onto a horizontal mounting face of the carrier bridge of the injection unit. The granulate hopper engages the guide plate with a hopper base equipped with a clamping lever which secures the hopper position. The guide plate extends to one side of the injection unit, where a discharge opening in the plate makes it possible to empty the hopper of its contents, prior to filling it with a charge of different raw material.
The possibility of emptying the granulate hopper, without removing it from the injection unit, speeds up the changeover from one raw material to another, thereby reducing machine downtime. The objective of reducing the changeover downtime to a minimum becomes very important in connection with injection molding machines which are used in short production runs, with frequent changeovers from the production on one part to the production of another part.
Such changeovers also require a corresponding exchange of injection molding dies in the die closing unit of the injection molding machine, including appropriate temperature conditioning of the new die assembly prior to installation and start of production.
In the past, the downtime caused by a changeover of die assemblies was of such magnitude that the simultaneous changeover of the raw material supply at the injection unit represented no additional downtime. Recently, however, through the use of program-controlled mechanized die handling devices, in conjunction with a die preconditioning system which allows for the insertion of a fully preconditioned die assembly, it has become possible to shorten the downtime required for an exchange of die assemblies to an absolute minimum. Accordingly, it has also become important to achieve a more rapid changeover from one raw material to another at the injection unit.